The hydraulic characteristics of the soil and aquifer system usually determine the design hydraulic loading rate of a site. In some instances, the nitrogen or BOD loading may control the area needed; however, the limiting design factor (LDF) for SAT systems is usually the infiltration/percolation rate. The design hydraulic loading rate is the measured clean water infiltration rate multiplied by a design factor. The design factor depends on the procedure used for measuring the infiltration rate, on the variability of the infiltration test results, on the variation in soil characteristics over the site, and on the conservatism of the designer. Design factors account for the cyclical (intermittent loading and drying) nature of SAT applications, the variability of site conditions and test measurements, and a long-term decrease in infiltration rates as a result of wastewater loadings. Typical design factors for different field tests are presented in Table 8.21.
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